The present invention relates to magnetic resonance imaging (MRI) and in particular local coils for use in transmitting radio frequency excitation signals and/or receiving magnetic resonance signals in magnetic resonance imaging.
Magnetic resonance imaging is used to generate medical diagnostic images by measuring faint radio frequency signals (magnetic resonance) emitted by atomic nuclei in tissue (for example, protons) after radio frequency stimulation of the tissue in the presence of a strong magnetic field.
The radio frequency stimulation may be applied, and the resulting magnetic resonance signal detected, using a xe2x80x9clocal coilxe2x80x9d having one or more single turn conductive xe2x80x9cloopsxe2x80x9d serving as antennas. The loops of the local coil are tuned to a narrow band, for example, 64 megahertz for a 1.5 Tesla magnetic field, strength magnetic field, and adapted to be placed near or on the patient to decrease the effects of external electrical noise on the detected magnetic resonance signal. The detected magnetic resonance signal may be conducted through one or more signal cables to the MRI machine for processing.
A local coil may incorporate multiple loops whose signals may be combined prior to being processed by the MRI machine, for example, in a quadrature type coil where perpendicular loops are combined with a 90xc2x0 phase shift. Alternatively, the signals of the multiple loops may be conducted independently to the MRI machine to provide for the so-called xe2x80x9cphased arrayxe2x80x9d detection.
An important characteristic of a local coil is the homogeneity of its field strength, the latter defined as the coil""s sensitivity to magnetic resonance signals when operated in a receive mode, and the strength of the coil""s transmission of radio frequency excitation signals when operated in the transmit mode. Homogeneity is particularly important for certain MRI procedures such as fat saturation where too much;or too little field strength may detrimentally affect the imaging process.
Field strength is a complex function of the design of the local coil and of the coil""s interaction with the patient. Homogeneity is often a compromise with other desirable coil characteristics including signal-to-noise ratio and selection of a coil shape.
The present invention provides a method of adjusting the field strength of different portions of loop by adding a shunting conductor that bypasses some current flow at one end of the loop. The shunt, in contrast to a resistive device, does not degrade the Q (quality factor) of the loop as would decrease the signal-to-noise ratio of the loop. By allowing control of the field strength independent of the coil geometry, the shunt permits field strength homogeneity to be increased or permits greater flexibility in other aspects of the coil design without significant loss of field strength homogeneity.
Specifically then, the present invention provides an MRI coil having a loop conductor split by a shunt conductor to divide a first loop portion from a second loop portion. The loop is tuned to a resonance frequency so that the current flow in the first and second loop portions are different.
It is thus one object of the invention to provide a means for controlling current flows within a loop to tune the field strength over different portions of the loop.
The MRI coil may include a matching network for conducting a signal related to the current flow in the first portion to an MRI machine.
Thus, it is another object of the invention to provide a field strength adjustment mechanism that permits a single tap point on the loop.
The area of the first loop portion may be different from the area of the second loop portion.
It is thus another object of the invention to allow altering the field strength profile of the loop by adjusting placement of the shunt conductor.
The MRI coil may include a patient support positioning the loop with respect to a patient so that the second loop portion is closer to the first loop portion of the loop and the proportion of current flow in the first and second loop portions may be adjusted to equalize the sensitivity of the first and second loop portions with respect to the patient.
Thus it is another object of the invention to provide a method of compensating for changes in field strength caused by variable separation distance between portions of the loop and the patient as may occur in certain desirable coil shapes.
The MRI coil may include multiple loops each split by a shunt conductor as described above.
It is thus another object of the invention to allow multi loop coils to have improved homogeneity using this technique.
The multiple loops may be placed adjacent to each other to form a tubular surface with the shunts parallel to the circumference of the tube, and in one embodiment, the tube may taper inward toward the center of the tube at one end of the tube.
Thus it is another object of the invention to allow construction of dome top tubular local coils with better homogeneity.
These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.